Technologies based on X-rays, gamma-rays and neutrons have been proposed to tackle this problem (Hussein, E., 1992, Gozani, T., 1997, An, J. et al, 2003). The most widely adopted technology is the X-ray scanner which forms an image of an item being examined by measuring the transmission of X-rays through the item from a source to a spatially segmented detector. X-rays are most strongly attenuated by dense, higher atomic number materials such as metals. Consequently, X-ray scanners are ideal for detecting items such as guns, knives and other weapons. However, X-rays provide little discriminating power between organic and inorganic elements. Using X-rays the separation of illicit organic materials such as explosives or narcotics from commonly found, benign organic materials is not possible.
An elemental identification system is being developed for the inspection of commodities shipped on pallets. The system called NELIS (Neutron Elemental Analysis System) utilises a 14 MeV neutron generator and three gamma ray detectors to measure induced gamma rays from the cargo (Dokhale, P. A. et al, 2001; Barzilov, A. P., Womble, P. C. and Vourvopoulos, G., 2001). NELIS is not an imaging system and is used in conjunction with an X-ray scanner to help identify gross composition anomalies.
A Pulsed Fast Neutron Analysis (PFNA) cargo inspection system has been developed (Gozani, T., 1997, Sawa et al., 1991) and commercialised through Ancore Corporation. The PFNA system uses a collimated beam of nanosecond-pulsed fast neutrons and the resulting spectrum of gamma rays is measured. The PFNA method allows the ratios of key organic elements to be measured. The nanosecond-pulsed fast neutrons are required in order to localise the specific regions contributing to the measured gamma-ray signal by time-of-flight spectrometry. In practice the technique is limited by the very expensive and complex particle accelerator, the limited neutron source strength and low gamma-ray detection efficiency and the resulting slow scan speeds.
Neutron radiography systems have the advantage of direct measurement of transmitted neutrons and are therefore more efficient than techniques measuring secondary radiation such as neutron-induced gamma rays. Fast neutron radiography has the potential to determine the line-of-sight ‘organic image’ of objects (Klann, 1996). In contrast to X-rays, neutrons are most strongly attenuated by organic materials, especially those with high hydrogen contents.
A fast neutron and gamma ray and radiography system has been developed by Rynes et al (1999) to supplement PFNA. In this system nanosecond-pulsed fast neutrons and gamma rays from an accelerator are transmitted through the object and the detected neutron and gamma ray signals are separated by arrival time. The resulting system is claimed to combine the advantages of both X-ray radiography and PFNA systems. However it is limited by the very expensive and complex particle accelerator.
Bartle (1995) has suggested using the fast neutron and gamma-ray transmission technique (Millen et al, 1990) to detect the presence contraband in luggage, etc. However this technique has not been used for imaging and its practical application to contraband detection has not been investigated.
Mikerov, V. I. et al, (2000) have investigated the possibility of fast neutron radiography using a 14 MeV neutron generator and luminescent screen/CCD camera detection system. Mikerov found that applications were limited by both the low detection efficiency of the 2 mm thick luminescent screen for fast neutrons and the high sensitivity of the screen to X rays produced by the neutron generator.
Neutron radiography systems using a 14 MeV generator and thermal neutron detection are commercially available (Le Tourneur, P., Bach, P. and Dance, W. E., 1998). However the fact that the fast neutrons are slowed down (thermalised) prior to performing radiography limits the size of the object being imaged to a few cm. No fast neutron radiography systems are commercially available that involve fast neutron detection.
Most work conducted with neutron radiography has been conducted in the laboratory using neutrons from nuclear reactors or particle accelerators that are not suited to a freight-handling applications (Lefevre, H. W, et al, 1996, Miller, T. G., 1997, Chen, G. and Lanza, R. C., 2000, Brzosko, J. S. et al, 1992).
To improve the ability of fast neutron radiography systems to provide discrimination between various organic materials, systems using multiple neutron energy sources, together with detectors with the means for distinguishing between the different neutron energies have been proposed (Chen, G. and Lanza, R. C., 2000, Buffler, 2001). The key drawbacks of these systems have been their reliance on complex, energy-discriminating neutron detectors and/or their use of sophisticated, high-energy accelerator-based neutron sources.
Perion et al. (Perion, 2000) have proposed a scanner using a high-energy (MeV) X-ray Bremsstrahlung or radioisotope source. By either modulating the average source energy by rapidly inserting and removing a low atomic number filter, or by measuring the energy of detected X-rays, it is possible to measure transmission through the object being scanned over two different X-ray energies, one where Compton scattering dominates and one where pair-production is significant. This information can be used to deduce the density and average atomic number of material in each pixel of the scan image. The main drawback of this scheme is the low contrast between different elements, even when very high energy X-ray sources are used. The cost of the Perion detector array would also be very high. Alternatively, Perion suggests that measurement of the transmission of both X-rays and neutrons (produced either directly in the Bremsstrahlung target or by inserting a neutron-producing filter) can yield similar information. The main disadvantage of this method is the low energy of neutrons produced via (gamma, n) reactions. This limits the ability of the neutrons to penetrate through thick cargoes and increases the difficulty in adequately detecting the transmitted neutrons. In particular, it is unlikely that the disclosed stacked scintillator detector would be able to distinguish neutrons in the presence of a much more intense X-ray beam. A disadvantage of both the dual energy X-ray and the X-ray/neutron schemes is that the X-rays and neutrons cover a wide range of energies. This means that it is not possible to model transmission using a simple exponential relation and that it is not straightforward to extract quantitative cross-section information that could be used for material identification.